Since the invention of the integrated circuit, the semiconductor industry has continuously sought to improve the density of integrated circuit components (transistors, diodes, resistors, capacitors, etc.). For the most part, improvements in density have come from reductions in feature size, allowing more components to be formed within a given area.
An essential tool for integrated circuit manufacturing is photolithography. The minimum feature size that can be resolved by a photolithography system, referred to as the critical dimension (CD), is proportional to the wavelength of light that is used. This has led to the development of lithography systems that use EUV light, which is light having a wavelength in the range from about 1 to about 100 nm. An EUV lithography system requires an EUV light source. A typical EUV light source forms a microplasma by shooting droplets of tin at a target on which the output of a CO2 laser is focused.
The microplasma emits radiation in all directions. It is highly desirable to collect that radiation as efficiently as possible and redirect it towards an illumination system of the photolithography tool. In order to keep the size of the collector's mirror within reasonable limits, the collector has to be mounted relatively close to the microplasma. As a consequence, the mirror tends to becomes contaminated by debris ejected from the plasma region. Contamination results in a loss of reflectivity, which eventually necessitates replacement of the collector. Replacing the collector requires a significant amount of downtime for the photolithography tool.